Resonant piezoelectric sensor

Abstract

A sensor for generating a signal in response of a variation at the tactile face. The sensor comprises superposed transmitting and receiving layers, which are provided with electrodes for the supply and pick-up of electrical signals to the transmitting layer and from the receiving layer respectively. With a high acoustic impedance of the boundary medium adjacent to the transmitting layer and a low acoustic impedance of the boundary medium adjacent to the receiving layer or vice versa, the sum of the thicknesses of the transmitting and receiving layers is an odd number of v/4f. With similar impedances, either low or high, of the said boundary media the aforesaid sum is an even number of v/4f. v is the propagation velocity of the acoustic vibration in the material of the transmitting and receiving layers and f the frequency of the signal supplied.

We claim: 
     
       1. Sensor comprising a transmitting layer and a receiving layer located thereon of piezoelectric material, which are provided with electrodes for supplying and deriving electrical signals to said transmitting layer or from said receiving layer respectively, wherein in order to obtain a standing wave in a unit of transmitting and receiving layer, a sum of the thicknesses of said transmitting and receiving layers is selected such that with a high acoustic impedance of a boundary medium adjacent to said transmitting layer and a low acoustic impedance of a boundary medium adjacent to said receiving layer or vice versa, said sum is an odd number of v/4f, and with similar impedances, either low or high, of the said boundary media said sum is an even number of v/4f, where v is a propagation velocity of acoustic vibration in a material of said transmitting and receiving layers and f is a frequency of a signal supplied and wherein said receiving layer detects a change of a resonance mode or resonance frequency of said sensor as a unit, said change being produced by a change of environmental conditions of said sensor and on its turn producing a change of sensed amplitude. 
     
     
       2. Sensor according to claim 1, wherein an outside of said transmitting or receiving layer is adjacent to a medium with a low or high fixed acoustic impedance and that said receiving or transmitting layer respectively is provided at the outside with a layer having a variable acoustic impedance, a variation of which is dependent on a physical quantity to be sensed. 
     
     
       3. Sensor according to claim 1, wherein outsides of said transmitting and receiving layers are provided with a layer having a variable acoustic impedance, a variation of which is dependent on a physical quantity to be sensed. 
     
     
       4. Sensor according to claim 3, wherein said layer having a variable acoustic impedance, comprises a porous or foamed material. 
     
     
       5. Sensor according to claim 1, wherein an outside of said transmitting or receiving layer is adjacent to a medium with a low or high fixed acoustic impedance and that said receiving or transmitting layer respectively is provided at the outside with a layer that couples acoustically to an object with variable efficiency, a variation of which is dependent on a physical quantity to be sensed. 
     
     
       6. Sensor according to claim 1, wherein outsides of said transmitting and receiving layers are provided with layers which coupled acoustically to a respective object and support with variable efficiency which variation is dependent on a physical quantity to be sensed. 
     
     
       7. Sensor according to claim 6, wherein an inside face of the layer having a variable coupling efficiency is roughened or corrugated. 
     
     
       8. Sensor according to claim 1, wherein with a variable acoustic impedance of the boundary medium at a receiving side and a sensor thickness of 1/4λ, 1/2λ, 3/4λ, or λ of an acoustic wave, thickness ratio of transmitting and receiving layers with a high acoustic impedance of the boundary medium at said transmitting layer is respectively very small, 1:1, 1:4 and 4:1 and with a low acoustic impedance of the boundary medium it is respectively 1:1, 1:2, 2:1 and 1:1 and with said boundary medium with a relatively low acoustic impedance and a sensor thickness of 1/2λ or λ, thickness ratio is respectively 2:3 and 1:1. 
     
     
       9. Sensor comprising superposed transmitting and receiving layers and a common electrode located between said transmitting and receiving layers, wherein other surfaces of said transmitting and receiving layers are provided with input electrode tracks and output electrode tracks respectively, said input electrode tracks extending parallel to each other and said output electrode tracks extending parallel to each other and perpendicular to the input electrode tracks, in which transmitted signals are supplied in sequence to said input electrode tracks and in which said output electrode tracks are connected to a read-out device. 
     
     
       10. Sensor according to claim 9, wherein a multiplexer is present via which said transmitted signals are derived from an electrical signal source. 
     
     
       11. Sensor according to claim 9, wherein said read-out device is formed by a multiplexer. 
     
     
       12. Sensor according to claim 9, wherein each output electrode track is connected via a detector circuit to an integrator circuit being reset after each scanning cycle, and wherein outputs of the integrator circuits are read out in parallel. 
     
     
       13. Sensor according to claim 9, wherein a tuning coil is connected in series to said input electrode tracks and/or to said output electrode tracks. 
     
     
       14. Sensor according to claim 9, wherein a tuning coil is connected in parallel to said input electrode tracks and/or to said output electrode tracks.